I. Field of the Invention
This invention relates to a procedure for obtaining improved yields and stability from halogenated rhenium-containing catalysts, especially halogenated platinum catalysts promoted with rhenium, or rhenium and another metal promoter, or promoters, in a reforming unit for reforming naphtha feeds.
II. Description of the Prior Art
Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas, or straight run gasolines, by increasing its aromatics content. In reforming, e.g., a naphthenic or paraffinic feed, is passed over a polyfunctional catalyst, or catalyst which contains an acidic component, e.g., a halide, and a metal hydrogenation-dehydrogenation component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably platinum, or platinum promoted with one or more additional metals, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Polymetallic reforming catalysts which include platinum and one or more promotor metals have now come into wide use. Reforming catalysts which contain platinum promoted with rhenium (e.g., U.S. Pat. Nos. 3,415,737 and 3,558,477), or both rhenium and iridium (e.g., U.S. Pat. Nos. 3,507,780 and 3,578,583), composited with porous inorganic oxide supports, notably alumina, are well known. In commercial reforming operations wherein such catalysts are employed, one or a series of reactors (usually three or four) constitute the heart of the reforming unit. Each reactor is generally provided with a fixed bed, or beds, of the catalyst which receive downflow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. During the "on-oil" portion of an operating cycle, a naphtha feed, with hydrogen, usually recycle hydrogen gas, is cocurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and the several catalyst-containing reactors of the series. The sequences of reforming reactions take place as a continuum throughout the series of staged reactors of the reforming unit. The product from the last reactor of the series is separated into a liquid fraction, and a vaporous effluent. The former is recovered as a C.sub.5.sup.+ liquid product. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process to minimize coke production.
On initially cutting oil (naphtha) into the unit, i.e., the beginning of the start-up period, gas make, or C.sub.4.sup.- hydrocarbon production, is usually high and C.sub.5.sup.+ hydrocarbon production relatively low. The catalyst during the start-up period is, even when operating conditions are carefully controlled, characterized by high hydrocracking activity which produces considerable C.sub.4.sup.- gas, especially methane, with consequent low C.sub.5.sup.+ liquid production. As the operating run continues, the production of C.sub.4.sup.- hydrocarbons decreases, and the production of C.sub.5.sup.+ hydrocarbons increases and lines out at production levels approximating a steady state operation, at which time the period of start-up is ended. The activity of the catalyst thereafter gradually declines during the remaining on-oil portion of an operating cycle due to the build-up of coke. Accordingly, during operation thereafter, the temperature of the process is gradually raised to compensate for the activity loss of the catalyst caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processes of this type, the oil must be cut out of the unit and the catalyst must necessarily be periodically regenerated by burning off the coke at controlled conditions. After regeneration, and reactivation of the catalyst a new on-oil cycle is begun.